Use global rand.Rand instance in mathrand.

common/data/rand/mathrand/mathrand.go

Index: common/data/rand/mathrand/mathrand.go
diff --git a/common/data/rand/mathrand/mathrand.go b/common/data/rand/mathrand/mathrand.go
index 58b48efbb418f80f5d26849655f072537549ded9..cc521ffe4171bfd49779b9e524d0534ae85adc24 100644
--- a/common/data/rand/mathrand/mathrand.go
+++ b/common/data/rand/mathrand/mathrand.go
@@ -16,6 +16,17 @@ import (
 
 var key = "holds a rand.Rand for mathrand"
 
+// globalRand is a global mathrand.Rand instance that is backed by a math/rand
+// instance. It is locking, and is safe for return as a default.
+var (
+	// globalRandBase is the NON-LOCKING global *rand.Rand instance.
+	globalRandBase = wrapped{newRand()}
+
+	// globalRand is a Locking wrapper around globalRandBase. globalRandBase MUST
+	// not be used without holding this lock.
+	globalRand = &Locking{R: globalRandBase}
+)
+
 func newRand() *rand.Rand {
 	return rand.New(rand.NewSource(rand.Int63()))
 }
@@ -46,7 +57,7 @@ func Get(c context.Context) Rand {
 
 	// Generate a new Rand instance and return it. Our callers expect this to be

[Vadim Sh., 2016/12/20 01:40:38]

this comment is stale now

[dnj, 2016/12/20 02:16:12]

Done.

 	// concurrency-safe.
-	return wrapLocking(wrapRand(newRand()))
+	return globalRand
 }
 
 // Set sets the current *"math/rand".Rand object in the context.
@@ -241,8 +252,9 @@ func WithGoRand(c context.Context, fn func(r *rand.Rand) error) error {
 		return r.WithGoRand(fn)
 	}
 
-	// No Rand is installed in our Context. Generate a single-use Rand instance.
-	// We don't need to wrap this at all, since the premise of this method is
-	// that the result is not safe for concurrent use.
-	return fn(newRand())
+	// Return our globalRandBase. We MUST hold globalRand's lock in order for this
+	// to be safe.
+	globalRand.Lock()
+	defer globalRand.Unlock()
+	return fn(globalRandBase.Rand)
 }